A pacemaker is a medical device for implant within a patient that provides electrical stimulation pulses to selected chambers of the heart. Such stimulation pulses cause the muscle tissue of the heart (myocardial tissue) to depolarize and contract, thereby causing the heart to beat at a controlled rate. Most pacemakers can be programmed to operate in a demand mode of operation, i.e., to generate and deliver stimulation pulses to the heart only when the heart fails to beat on its own. To this end, the pacemaker senses cardiac activity, i.e., heart beats, and if the heart beats do not occur at a prescribed rate, stimulation pulses are generated and delivered to an appropriate heart chamber to force the heart to beat.
Proper operation of a pacemaker presupposes that stimulation pulses generated by the pacemaker effectuate capture. Capture refers to the ability of a given stimulation pulse generated by a pacemaker to cause depolarization of nearby myocardial tissues, i.e., to generate an evoked response (ER) and to cause heart muscle to contract. Failure of a pulse to effectuate capture (i.e. no response is evoked) is referred to as a “loss of capture” or LOC. While many factors influence whether a given stimulation pulse effectuates capture, a principal factor is the energy of the stimulation pulse. The energy of the stimulation pulse, in turn, is determined by the amplitude and width (or duration) of the stimulation pulse generated by the pacemaker and the electrical resistance of the pacemaker system/tissue interface circuit. Advantageously, in a programmable pacemaker, both the amplitude and pulse width of the stimulation pulse are parameters that may be programmably controlled or set to a desired value. Herein, the term “magnitude” is used to generally refer to the energy of pulse. Magnitude may be adjusted by changing either or both pulse amplitude or pulse width.
Stimulation pulses may be delivered either in a bipolar mode or a unipolar mode. The term “bipolar” is used herein to refer to a mode wherein both of the electrodes used to deliver the stimulation (or sense cardiac signals) are located on or in the heart of the patient. One of the electrodes is used as a cathode (negative pole) and the other is used as an anode (positive pole). In some cases, both the cathodal and anodal electrodes are located on or within the same heart chambers, such as the left ventricle (LV). In other cases, the two electrodes of the bipolar pair are located on or within different chambers. For example, the cathode may be on the LV, whereas the anode may be in the right ventricle (RV). This type of bipolar stimulation can also be referred to as cross-chamber stimulation. The term “unipolar” is used herein to refer to a mode wherein only one of the electrodes used to deliver the stimulation (or sense cardiac signals) is located on or in the heart of the patient. This electrode is usually the cathode. The device housing or “can” is used as the other electrode (typically the anode.) Note that, in the literature, cross-chamber forms of stimulation/sensing are sometimes referred to as “unipolar” but, to avoid confusion herein, unipolar is reserved for stimulation/sensing modes where only one of the electrodes is located on or in the heart.
Conventionally, bipolar stimulation pulses are set to a magnitude sufficient to effectuate capture only at the cathode (negative pole) of the pair of electrodes used to deliver the stimulus. This is referred to herein as “cathodal-only”stimulation. A higher pulse magnitude is typically required to additionally effectuate capture at the anode (positive pole) of the electrode pair. This is referred to herein as “anodal/cathodal” stimulation. Usually, anodal/cathodal stimulation is not warranted, and the additional energy required to achieve anodal capture as well as cathodal capture would unnecessarily burden the energy resources of the device, possibly reducing battery life. However, concurrent anodal/cathodal capture may be desirable in some cases since it achieves capture at two separate sites within the heart and hence may achieve synchronized myocardial contractions at two locations.
Many pacemakers now include automatic stimulation threshold search systems that, following implant of the pacemaker, automatically determine a capture threshold and set the stimulation pulse amplitude accordingly, but these systems typically apply only to cathodal stimulation, i.e. the capture threshold is the threshold for cathodal-only stimulation. Herein, the capture threshold (or minimum pulse energy) sufficient to evoke cathodal capture only is abbreviated CAPCATHODE. Likewise, many pacemakers include automatic capture verification systems which, following delivery of stimulation pulses, automatically verifies that the pulses are captured (i.e. an ER is produced) and takes steps if capture is lost, but these systems also typically apply only to cathodal stimulation.
It would be desirable to provide improved techniques for determining capture thresholds and verifying capture that additionally apply to concurrent anodal/cathodal capture. Aspects of the present invention are directed to this end. For background regarding anodal capture, see, e.g. techniques described in U.S. Patent Application 2010/0121396 of Gill et al., entitled “Enhanced Hemodynamics through Energy-Efficient Anodal Pacing” and U.S. patent application Ser. No. 11/961,720, filed Dec. 20, 2007, of Snell et al., entitled “Method and Apparatus with Anodal Capture Monitoring.”
Other aspects of the present invention are directed to exploiting concurrent anodal/cathodal stimulation techniques for use with MSLV pacing. MSLV pacing aims to improve intra-LV synchrony and overall response to CRT by initiating a linear waveform of depolarization using the various electrodes of a multi-polar LV lead. State-of-the-art CRT devices typically offer two independent LV pulses (LV1 and LV2) to capture two LV electrode locations (i.e., dual-site capture) using cathodal capture. For example, when using a quad-pole LV lead having a distal tip electrode (D1), a first intermediate ring electrode (M2), a second intermediate ring electrode (M3) and a proximal ring electrode (P4), the first LV pulse (LV1) may be delivered using D1-M2 to achieve cathodal capture at D1 while the second LV pulse (LV2) is delivered using P4-M3 to achieve cathodal capture at P4. This requires two pulses to achieve dual-site capture. Within such devices, to achieve capture at all four sites via cathodal-only capture would require delivering a second set of LV1 and LV2 pulses configured to achieve capture at the other two sites (M2 and M3.) This requires more energy and results in an inevitable delay between the first two LV1 and LV2 pulses and the second two LV1 and LV2 pulses, preventing simultaneous or concurrent stimulation at all four sites.
An alternative technique for achieving dual-site capture would instead utilize a single pulse set to a higher magnitude sufficient to achieve both anodal and cathodal capture. For example, a larger magnitude LV1 pulse could be delivered using D1-P4 to achieve cathodal capture at D1 and anodal capture at P4. Depending upon the pulse magnitude, this might be actually consume less energy than using two LV pulses (LV1 and LV2) set for cathodal-only capture. Still further, by exploiting anodal/cathodal capture, stimulation may be delivered at four sites concurrently. For example, a large magnitude LV1 pulse could be delivered using D1-M2 to achieve cathodal capture at D1 and anodal capture at M2 while a large magnitude LV2 pulse could be delivered using M3-P4 to achieve cathodal capture at M3 and anodal capture at P4. Again, depending upon the pulse magnitude, this might be actually consume less energy than using two pairs of pulses—a first LV1, LV2 pair followed closely by a second pair LV1, LV2 pair—set for cathodal-only capture. Whether a reduction in energy consumption can be achieved within a given patient when using concurrent anodal/cathodal stimulation will likely depending on whether the anodal/cathodal capture threshold can be determined precisely and whether concurrent anodal/cathodal capture can be efficiently verified.
Accordingly, it would be desirable to provide improved techniques for determining capture anodal/cathodal thresholds and for verifying anodal/cathodal capture for use with MSLV pacing and various aspects of the invention are directed to this end, as well.